1. Field of the Invention
This invention relates generally to radiation detectors and more particularly to semiconductor photodetectors having high sensitivity at low light intensity.
2. Description of the Prior Art
The possibilities of increased communications capacity and smaller size, compared to present communications systems, have led to serious consideration of the feasibility of optical communications systems. For the systems presently contemplated using glass transmission lines, a photodetector, necessarily compatible with the specialized size, cost, frequency response and sensitivity requirements imposed by other system components, is needed to convert optical energy into electrical current. Semiconductor radiation detectors appear able to satisfy system requirements imposed on the photodetectors.
In semiconductor radiation detectors, including those sensitive to optical radiation and called photodetectors, the incident radiation interacts with the semiconductor material and creates free charges, i.e., electron-hole pairs, which can provide evidence of the presence of incident radiation when the radiation detector is suitably connected to an external circuit and a current that is proportional to the intensity of the incident radiation flows. Efficient detection of the free charges is facilitated by use of a reverse biased p-n junction. Important features of this junction include an absorption region in which the incident radiation interacts with the semiconductor material and a depletion region having a high electric field formed by immobile positively charged donor atoms and negatively charged acceptor atoms on the n and p sides of the junction, respectively. Photodetectors of this type considered for optical communications use include p-i-n photodiodes, avalanche photodiodes and phototransistors.
All prior art devices of the above types have drawbacks for optical communications use. Optical transmission lines carry light of low intensity and the current from a p-i-n photodiode is correspondingly small and requires amplification which, because of the thermal noise necessarily associated with the amplifier, severly limits the ultimate sensitivity attainable. An avalanche photodiode eliminates some of this constraint on sensitivity with internal amplification. However, the internal amplification requires both high voltage and either temperature compensation or automatic gain control to prevent the detector output signal from varying with changes in the ambient temperature. Phototransistors have not hitherto been as seriously considered for use in optical communications systems as have p-i-n and avalanche photodiodes because their response times have been thought too slow for the high data rates, e.g., 50 Mbit/sec, contemplated and their high capacitances have been thought to impose too severe a limitation on the attainable noise performance. These limitations have made phototransistors less desirable than either p-i-n or avalanche photodiodes in optical communications systems.